Acute rejection or side effects occur in nearly half of all transplant patients receiving immunosuppression. During our studies with regimens of (SRL) sirolimus+cyclosporine/tacrolimus (CsA/TAC), cytokine and costimulatory cell surface proteins (biomarkers), have demonstrated sensitivity to clinically relevant immunosuppressive drug concentrations in mitogen-stimulated peripheral blood lymphocytes (PBL) from normal and transplanted human subjects. Effect: concentration (pharmacodynamic, PD) relationships between biomarkers and drug concentrations also indicate ability to measure single- and multiple-agent effects within combination regiments, and to predict the amount of drug needed to inhibit a certain amount of biomarker, both for patient populations and individuals. However, prior to use as measures of immunosuppressive effect, the amount of biomarker inhibition associated with clinical conditions representing insufficient, excessive and adequate immunosuppression must be known. This may define safe amounts of drugs for children, who experience a higher incidence of life-threatening complications of immunosuppression such as post-transplant lymphoproliferative disorder. Therefore, the specific aim of this project is to measure biomarker expression during a planned pharmacokinetic (PK) evaluation of a SRL+TAC regimen in 40 children with liver transplants, relate it to amount of drug, and to determine whether the occurrence of acute rejection, side effects and stable post-transplant course can be related to threshold levels of biomarker inhibition or the amount of drug associated with such thresholds. During a sponsored clinical trial of SRL+TAC, our proposal will manage biomarker data as follows: 1. Measure mitogen-stimulated expression of the cytokines IL-2, TNF-alpha and IFN-gamma in T-cells, and of costimulatory proteins CD54 (intercellular adhesion molecule-1), CD86 (B7.2) and CD95 (Fas antigen) in B-cells, and the proliferative response of lymphocytes to donor antigen. This will be performed during PK studies planned in the clinical trial, and additionally, during rejection, side effects, and at 12, and 24 month after transplantiation. 2. PD modeling to predict biomarker thresholds or drug concentrations associated with them, which are related to the occurrence of acute rejection, side effects, and the stable post-transplant course. Potential benefits may include customized regimens in the future, and decreased complications in one-half of the nearly 40,000 new transplant recipients of solid organ and bone marrow grafts, each year.